A number of iron-containing compounds of biological-origin were found to replace synthetic iron chelates in catalyzing the peroxidation of amino acids. Detailed studies with a system comprised of hydrogen peroxide, bicarbonate, hemin or ferritin was found to mimic the bicarbonate-dependent oxidation of leucine by the more conventional Fenton reagent (hydrogen peroxide plus Fe(llI) plus EDTA). A complete analysis of the reaction products established that leucine oxidation occurs by three independent pathways: (1) Oxidative deamination to form alpha-ketoisocaproic acid and ammonia: (2) Oxidative decarboxylation-deamination to form isovaleraldehyde, bicarbonate and ammonia; (3) Oxidation to the isovaleraldehyde oxime and carbon dioxide. Material and oxidation- reduction balances demonstrate that these three reactions and the amino acid independent iron-catalyzed decomposition of hydrogen peroxide to oxygen account for 90-95% of the hydrogen peroxide and leucine which are consumed. By means of difference spectroscopy, it was shown that there is a time-dependent interaction between Fe(III), oxygen and amino acid. This together with results of earlier studies showing that amino acid oxidation is insensitive to radical scavengers supports the conclusion that amino acid oxidation involves in situ generation activated oxygen species within the Fe(III)- or Fe(II)-amino acid complex, and is not the result of a subsequence free-radical attack of the amino acid by oxygen radicals generated via an amino acid independent mechanism.